The lidar ratio (the ratio of the extinction coefficient and the back scattering coefficient) is an important optical parameter.A novel method of Mie scattering lidar to determine averaged lidar ratio in cirrus clouds is proposed.This method takes two parameters of cloud bottom aerosol extinction c oefficient and the optical thickness of the cirrus as constraints,constructing the nonlinear equation tha t contains two unknow parameters which are the aerosol extinction coefficient of cirrus′s top and the lidar rati o of cirrus.Then,by the use of the particle swarm optimization algorithm,the extinction coefficient of the cirrus ′s top and the lidar ratio of cirrus clouds are acquired.The experiment based on the proposed method is carried on, using the real return signals of the ground-based lidar.It′s turned out that the proposed approach is effecti ve,with the advantages of fewer iterations and faster convergence of iteration.Therefore,it has a good appl ication prospect.